1. Field of the Invention
The present invention relates to a photonic microwave mixing apparatus and method thereof by using, particularly, period-one nonlinear dynamics of semiconductor lasers.
2. Description of the Related Art
According to the statistics released by a leading telecommunication manufacturer, Cisco Systems, Inc, the data traffic of the global wireless communication has reached an average close to 3.7 million terabytes per month in 2015 and will continue to grow at a compound annual rate of 53% in the following years. In order to meet such a huge data traffic demand, telecommunication operators and manufacturers have proposed two solutions. On one hand, they have proposed to use high-frequency microwave signals as carriers (for example, Samsung and Nokia have proposed to use 28 and 70 GHz, respectively) for the next-generation wireless communication systems, such as 5G or beyond, to provide a communication bandwidth of 1000 times more than that of the current systems. On the other hand, they have proposed to adopt a new wireless communication access network architecture, the so-called radio-over-fiber network, which combines the advantages of both wireless communication and optical fiber communication to significantly enhance the communication capacity and also to considerably expand the communication coverage for the next-generation wireless communication systems.
As opposed to the present wireless communication access network, more than 80% of the microwave signal processing functionalities in the radio-over-fiber network will be carried out at central offices (or baseband processing offices) instead of remote base stations. Key microwave signal processing functionalities include (1) how to first convert low-frequency microwave signals to high-frequency microwave signals, a process called frequency upconversion, at central offices and next distribute the frequency-converted microwave signals through optical fibers to remote base stations for wireless radiation using antennas for downlink transmission, and (2) how to convert high-frequency microwave signals, which are first received by antennas at remote base stations and next transmitted to central offices through optical fibers, to low-frequency microwave signals, a process called frequency downconversion, at central offices for data retrieval and analysis using photodetectors with narrow bandwidth, low cost, and high output power for uplink transmission. These two key microwave signal processing functionalities can be achieved using photonic microwave mixing apparatuses. Compared with electronic microwave mixing apparatuses, photonic microwave mixing apparatuses enable frequency upconversion and downconversion over a broad spectral range, block the interaction between frequency-to-be-converted microwave signals and microwave local oscillators, and avoid electromagnetic interference. Consequently, the needs for high-frequency electronic microwave devices and equipment are largely reduced and the restriction on high-frequency upconversion or downconversion due to the limited bandwidth of electronic devices are considerably relaxed.
Three commonly adopted photonic microwave mixing apparatuses and methods for frequency upconversion and downconversion are briefly described as follows:
(1) Dual-series Modulators:
    This method utilizes an optical modulator to superimpose a microwave signal at f0, generated by an electronic microwave local oscillator, onto an optical signal carrying a frequency-to-be-converted microwave signal at fm in order to generate a wave-mixing optical signal. An optical filter is applied to select the desired optical frequency components, which are separated from each other by |fm−f0|, of the wave-mixing optical signal. An optical signal carrying a frequency-converted microwave signal at |fm−f0| is therefore obtained, achieving microwave frequency conversion from fm to |fm−f0|. One key advantage of this method is that the frequency-converted microwave signal has high spectral stability and purity. However, since an electronic microwave local oscillator is required, high-frequency microwave mixing may not be available due to the bandwidth limitation of the electronic microwave local oscillator. In addition, since a high output power from the electronic microwave local oscillator is required for high conversion efficiency and an optical power amplifier is needed to compensate for the significant power loss after the process of the optical filtering, the system power consumption is considerably high.(2) Dual-parallel Modulators:    This method adopts an optical modulator to superimpose a microwave signal at f0, generated by an electronic microwave local oscillator, onto a continuous-wave optical signal. This optical signal is used to destructively interfere with another optical signal carrying a frequency-to-be-converted microwave signal at fm so that the optical carriers of both optical signals are removed or suppressed. An optical signal carrying a frequency-converted microwave signal at |fm−f0| is obtained accordingly, achieving microwave frequency conversion from fm to |fm−f0|. Key advantages of this method include that the frequency-converted microwave signal has high spectral stability and purity, no optical filter is required, and the optical modulation depth of the resulting optical signal can be adjusted to enhance the conversion efficiency. However, one major disadvantage of this method is that a highly precise optical phase difference between the two interfering optical signals is needed, which requires high stability of the conversion system against ambient variations and system adjustments. In addition, since a high-power electronic microwave local oscillator is required for high conversion efficiency, not only high-frequency microwave mixing may not be available due to the bandwidth limitation of the electronic microwave local oscillator, but also the system power consumption is considerably high.(3) Cross-gain Modulation:    This method takes advantages of the cross-gain modulation effect that happens inside a semiconductor optical amplifier when two optical signals carrying microwave signals at fm and f0, respectively, are simultaneously sent through the semiconductor optical amplifier in order to generate a wave-mixing optical signal. An optical filter is applied to select the desired optical frequency components, which are separated from each other by |fm−f0|, of the wave-mixing optical signal. An optical signal carrying a frequency-converted microwave signal at |fm−f0| is therefore obtained, achieving microwave frequency conversion from fm to |fm−f0|. One key advantage of this method is that the frequency-converted microwave signal has high spectral stability and purity. However, since an electronic microwave local oscillator is required, high-frequency microwave mixing may not be available due to the bandwidth limitation of the electronic microwave local oscillator. In addition, since a high output power from the electronic microwave local oscillator is required for high conversion efficiency and an optical power amplifier is needed to compensate for the significant power loss after the process of the optical filtering, the system power consumption is considerably high.